In the manufacture of gypsum board, it is common practice to incorporate air bubbles into gypsum slurries by the addition of foaming agents. The benefits of entraining air bubbles into the gypsum slurries include:
(a) reduced amount of water needed to render the gypsum slurry adequately fluid thereby minimizing the energy needed to drive off any excess water; PA1 (b) reduced density of the hardened gypsum products; and PA1 (c) reduce cost of gypsum board.
Gypsum is mainly composed of calcium sulphate. Foaming agents, and particularly alkyl sulfates and alkyl ether sulfates are sensitive to calcium ions. And, in the actual manufacture of gypsum board, the foaming agent is exposed to a constant flow of calcium ions. Thus, it is important for the foaming agent which is to be used in the manufacture of gypsum board to be able to produce copious amounts of foam in saturated calcium sulfate solutions where the presence of solid gypsum particles are predominant.
Therefore, it is important to test gypsum foaming agents in an environment which is as close as possible to that which such agents will encounter in the actual manufacturing of gypsum wallboard. The method of evaluation used in this invention is one in which the performance of the foaming agents is determined by their ability to entrain air into gypsum slurries. As will be illustrated, the foaming agents disclosed herein show substantial enhanced performance, as measured by their ability to entrain air into gypsum slurries, than foaming agents disclosed in the prior art.
Cukier, U.S. Pat. 4,156,615 issued May 29, 1979 (now assigned to Stepan Canada, Inc., assignee of the present invention) discloses a salt of an ethoxylated and sulfated primary linear alcohol. This alkyl ether sulfate has a general formula CH.sub.3 (CH.sub.2).sub.x CH.sub.2 (OCH.sub.2 CH.sub.2).sub.4 OSO.sub.3 M wherein the average value of x can range from 8.5 to 9.5 with molecular weights ranging from 121 to 135 and the values of x lying in the range of 8 to 10 (molecular weights lying in the range of 114 to 143) constitute at least 90 percent by weight of the active ingredient. The average value of y lies from 1.5 to 2.5, and M represents either sodium or ammonium ions. Cukier's preferred average value of x is 9 (molecular weight of 128) and the preferred average value of y is 2.2. This alkyl ether sulfate will hereinafter sometimes be referred to as Cukier, Sample A.
Green et al., U.S. Pat. No. 4,678,515 issued Jul. 07, 1987 (now assigned to Stepan Company) discloses foaming agents with improved foaming properties, comprised of a mixture of about 89 to 67 percent by weight of an alkyl ether sulfate of the type disclosed by earlier referenced Cukier, U.S. Patent '615, with about 11 percent to 33 percent by weight of a second alkyl ether sulfate with the Cukier general formula, wherein x ranges from 12 to 14 (molecular weights ranging from 179 to 199) and the average value of y ranges from 1 to 3. The teachings of Green et al. result in mixtures of alkyl ether sulfates having average values of x lying in the range from 9.4 to 10.4, (molecular weights ranging from 134 to 148) and average values of y ranging from 2.0 to 3.5. As previously discussed, the method of evaluating foaming agents is of substantial importance. In Green et al., U.S. Patent ' 515, performance was measured on the basis of the foam generated and the foam stability produced by a surfactant in an environment consisting of a saturated gypsum solution from which solid particles of gypsum had been removed. This is different from the environment which is present in the actual manufacture of gypsum board. The determining factor in the performance of a foaming agent is the ability of that foaming agent to be able to foam and entrain air in a gypsum slurry where a saturated solution of calcium sulfate, as well as solid gypsum particles, are always present. In an environment where there is a fixed amount of calcium ions, and no solid gypsum to replace those ions which may react with the foaming agent, there exists an excessive amount of foaming agent in such system, (the environment used by Green et al., U.S. Patent '515), and potential for inaccurate results may be present.
A more accurate test for determining the performance characteristics of a foaming agent is one which determines the ability of such agent to entrain air into a gypsum slurry while emulating conditions encountered in actual manufacture of gypsum products. Such a test, designated hereinafter as the Gypsum Block Test, was developed during attainment of this invention and results therefrom show that the foaming agents disclosed by this invention have a greater capacity to entrain air into a gypsum slurry than the surfactants disclosed by the earlier referenced Cukier, U.S. Patent '615, and/or Green et al., U.S. Patent '515.
It should also be noted that neither prior art teaching is concerned with the oxyethylene distribution. In accordance with the principals of the invention, while the molecular weight of R must be from about 169 to 91, changes within those parameters do not greatly affect the performance of the foaming agent. However, the oxyethylene distribution is a very important criteria. Changes in distribution greatly affect the performance of the foaming agent, even when the average value of y remains constant.
An important property of foaming agents especially in the manufacture of gypsum wallboard is their ability to be completely and rapidly dissolved in water, before they are generated into a foam to be incorporated into the gypsum slurry. Failure to completely and rapidly dissolve will result in gel-like particles being carried into the gypsum slurry. The processes of wallboard manufacture can be pictured as the creation of a "sandwich" of gypsum between paper covers. When gel particles are contained in the slurry and the slurry is subsequently dried to remove water in the setting of the gypsum, the gel particles result in voids where the gypsum filed to fill the locations. At the paper surface such a voids can even result in rupture of the paper. Gypsum wallboard with large voids is unacceptable.
Typically, manufacturers of gypsum wallboard purchase foaming agents in bulk quantities rather than drums an store such product in tanks for considerable and extended periods of time. Typical carriers or solubilizers such as low molecular weight alcohols such as methanol, ethanol and isopropanol in foaming agents present problems related to their relatively high vapor pressures. The high vapor pressures of these alcohols usually results in loss or evaporation of the carrier during storage. The foaming agent then very often exhibits poor solubility in water and can result in the preparation of a gypsum wallboard product that has voids and is, therefore, unacceptable. Moreover, because of their low vapor pressure and high flammability, special requirements and precautions including labeling must be observed which contribute to increased costs.